All of the comments above are valuable if you don't have a sales strategy with a clear and compelling USP. If you've been selling into small/mid size accounts and now want to swim upstream, the sales process is different as are the account planning process and how account plans are used.
Identifying your top targets requires answers to many of the questions above. If they are already identified, then it is important to understand the personas of the people you are selling to, why/how they buy, and massage your marketing messages to align with what is important to them.
The account planning process needs to begin with a structure. What are the elements you need to know about the account, e.g. organizational structure, political mapping and alignment, account's key strategic initiatives, competitive positioning, cthe strategy of how you want to pursue the account, the tactics that will be used to execute the strategy, opportunity mapping(which products/services fit), resources required. Once you have a format, do strategy sessions within the company with representation from cross-functional teams might help the sales person to refine their plan. The account plan is a roadmap of how they are going to penetrate the account. It should be created and updated either bi-annually or annually, and becomes reference for the account manager who is responsible.
Account Planning is one of the top 15 sales processes needed to create a world class sales team. If you'd like to learn more, please reach out to me on LinkedIn.
Please feel free to contact me if you'd like to see some of the formats I've used.
It is very important to teach sales people that the goal is not to complete account plans as an administrative exercise - the activity holds importance because when done correctly it serves to development territories, specific accounts, verticals, etc.